Well, I think a lot of what guides this thing is science.
Political science. Not going into that.
My thinking:
Age is an arbitrary number. Why 65? Why not 50?
Nobody will explain that.
Regardless, I'll be in 4 different countries, with who knows what variants, and then on a cruise ship, with who knows whose from where or where they've been.
Science.
64 is the same as 65 is the same as 66. 65 is arbitrary.
So is 12. So is 18.
It's silliness.
Yes, it's arbitrary. But it's also not.
We routinely break things into groups. 65 is a common one because it's "retirement" age. For health purposes we often use 50-64 and 40-49 because those age ranges tend to have similar health problems, yet we then treat 18-39, a 21 year gap, as a single group because those ages typically have very few age-related health issues at all, so they fit into one statistical group easily. And anything <18 is usually treated much differently because testing of medication/vaccine/etc for someone who is still in their prime growth stage may have different effects than it does for someone fully grown, and also because they're minors and not legally given responsibility for themselves, so they're treated different.
Obviously these are big categories that have levels of arbitrariness. If you're 64, does that mean that you should always listen to the 50-64 recommendation if you're in particularly poor health and your general level of health is closer to that of a 75 year old? If you're 66, does that mean you should always listen to the 65+ recommendations if you're fit, healthy, and have no chronic conditions? Obviously you have to take control of your own health.
But if the official recommendation is that people 65+, people who are immunocompromised or have significant comorbidities get the booster, I think that as a relatively healthy and fit 43 year old, I don't see any particular reason I'd need it. I might want it if I were 64 and healthy. But not really that important to me at 43.